According to initial police reports, at least two were killed and dozens injured when the blasts occurred, just after the four-hour mark of the renowned race. 

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Markey's primary opponent, Rep. Stephen Lynch (D-Mass.), said in a statement that he was "terribly saddened" by the news of the explosions.

"Right now we need to let the trained emergency personnel do their jobs to ensure that there are no other threats, and that we can get a better sense of what happened," he said. "No matter who or what is responsible, this is a terrible tragedy for our city and our nation. Our thoughts and prayers are with those who have been injured today."

Markey and Lynch will face off in the April 30 Democratic primary, ahead of a June 25 general election. The Senate seat came open when former Sen. John KerryJohn Forbes KerryFor the sake of national security, Trump must honor the Iran deal Bernie Sanders’s 1960s worldview makes bad foreign policy DiCaprio: History will ‘vilify’ Trump for not fighting climate change MORE (D) resigned earlier this year to become secretary of State.